For most, the major concerns are cosmetic ones. But an important risk is often overlooked: the quality and safety of the lenses.

Researchers found that nearly half of prescription glasses they bought online either contained the wrong lenses or didn't meet the standards for impact testing, meaning the lens can crack or shatter, according to a small study published last year in the journal Optometry.

The researchers ordered a total of 200 pairs of spectacles from the 10 most popular online retailers; they received and evaluated 154 pairs.

In some cases, they received single vision lenses instead of multifocal — an unheard of mix-up, said Karl Citek, a professor of optometry at the Pacific University College of Optometry and lead author of the study. Lens treatments such as anti-reflection coatings were either incorrectly added or omitted. In nearly a quarter of the spectacles, at least one lens failed impact testing, which is required because prescription eyeglasses are classified as medical devices by the Food and Drug Administration.

"A lens might be slightly off in the optics; if you don't have someone verify (the prescription) you might not know it's wrong," said Citek who does not recommend buying eyeglasses online. Vendors generally don't call your doctor to verify prescriptions, but even that "can't ensure that the lenses are safe since a doctor has no way of assessing impact resistance for finished eyewear," said Citek.

Still, online purchases are growing, mainly because glasses are so darn expensive.

According to the Vision Council, a trade group for the optical industry, 1.9 million pairs were bought online in 2010, about 2.8 percent of the total 66.8 million pairs bought in the U.S.

The bespectacled say it's liberating to be able to buy a $50 pair of eyeglasses, or buy five pairs for the price of one. Some sites offer virtual fittings; simply upload a photo of yourself and superimpose the frames you like.

Or, try them on the old fashioned way. The Warby Parker Home Try-On Program allows you to pick five pairs of glasses to keep for five days. Shipping is free both ways.

If you decide to try your luck online, Citek recommends asking your doctor to verify the prescription and to adjust the frame once you've received the glasses. Doctors may charge for this. It's also important to check the company's warranty and return policy.

Finally, "never order children's eyewear online," he said. The study finding that one in four pairs of children's eyewear failed impact resistance testing is "unconscionable," said Citek.

1 in 4 pairs of children's eyewear failed impact resistance testing, according to a small study published last year in the journal Optometry. The study also showed that almost half of the glasses surveyed didn't provide the stated prescription or had issues with impact resistance testing.

Catasauqua police officer Scott M. Rothrock had already been stabbed once in the chest with a 13-inch butcher knife and was trying to block other stabbing attempts as he lay on his back in a snow bank in east Allentown last month.